Assigning Oxidation Numbers

Oxidation-reduction reactions (redox reactions)
are reactions in which electrons are lost by an atom or ion in one reactant
and gained by an atom or ion in another reactant. Although electrons are
gained and lost in these reactions, the balanced equation for a redox
reaction does not show the electrons that are being transferred. In order
to tell whether a redox reaction has occurred or not, we need a way to
keep track of electrons. The best way to do so is by assigning oxidation
numbers to the atoms or ions involved in a chemical reaction.

Oxidation numbers
are hypothetical numbers assigned to an individual
atom or ion present in a substance using a set of rules. Oxidation
numbers (or oxidation states as they are
also called) can be positive, negative, or zero. It is VERY IMPORTANT
to remember that oxidation numbers are always
reported for one individual atom or ion and not for groups of atoms or
ions.

The following rules are used to assign oxidation
numbers. Chem 1115 students will have these rules available on exams.
Chem 1215 students must memorize these rules.